Controlling a multi winding set permanent magnet electrical machine

1. A method for controlling a multi winding set permanent magnet electrical machine in case of a fault in a first group of winding sets leaving a second group of winding sets functional, the method comprising: determining a value of a torque generated by all winding sets; and controlling values of currents of the second group of winding sets based on the value of the torque to reduce a damaging torque and/or a torque oscillation occurring and/or to brake the multi winding set permanent magnet electrical machine, wherein the controlling values of currents of the second group comprises controlling values of a d-component and/or a q-component of the currents of the second group of winding sets based on the value of the torque, wherein the controlling the values of the d-component of the currents of the second group of winding sets comprises setting a fault reference value of the d-component of the currents of the second group of winding sets based on a sign of the value of the torque, wherein if sign of the value of the torque is negative, the fault reference value of the d-component of the currents is set to a negative value supported by converters connected to the second group of winding sets.

2. The method according to claim 1 , wherein if sign of the value of the torque is positive, the fault reference value of the d-component of the currents is set to a value being smaller than the absolute value of the negative value.

3. The method according to claim 1 , wherein the controlling the values of the q-component of the currents of the second group of winding sets comprises: deriving a reference of a counteracting torque counteracting a torque oscillation based on an oscillation portion using a band/high pass filter of the determined value of the torque; and deriving a fault reference of the q-component of the currents from the reference of the counteracting torque.

4. The method according to claim 1 , wherein the controlling values of the q-component of the currents of the second group of winding sets comprises: deriving a reference of a counteracting braking torque counteracting a torque oscillation and braking the machine, based on an oscillation portion of the determined value of the torque and a braking torque reference; and deriving a fault reference of the q-component of the currents from the reference of the counteracting braking torque.

5. The method according to claim 4 , wherein the braking torque reference is determined such that the reference of a counteracting breaking torque is below a maximally possible torque that can be generated by the second winding sets, and/or converters connected to the second winding set.

6. The method according to claim 4 , wherein deriving a fault reference of the q-component of the currents further comprises applying a limitation on torque and/or current.

7. The method according to claim 1 , wherein determining a value of the torque comprises: measuring the torque; and/or measuring, using at least one accelerometer, a vibration of at least one component of the machine or wind turbine or a speed of the rotor; and deriving the value of a torque from the measured vibration or the measured speed; and/or estimating the torque by using one or more observers of speed of rotor and/or electromagnetic torque of the second group of winding sets.

8. The method according to claim 1 , wherein the second group of winding sets is connected respectively to a second group of converters, wherein the controlling values of currents of the second group of winding sets comprises controlling the second group of converters.

9. The method according to claim 1 , wherein, before the fault, the first group of winding sets is connected respectively to a first group of converters, and after detection of the fault, the first group of winding sets are disconnected from the first group of converters.

10. The method according to claim 1 , wherein the first group of winding sets is connected to a first group of converters and wherein the second group of winding sets is connected to a second group of converters, each one of the first and/or the second group of converters comprises a generator side converter, a DC-link and a grid side converter, wherein before detection of the fault, the generator side converters perform voltage control of the DC-link; wherein after detection of the fault, the generator side converters switch to torque control; wherein after detection of the fault, the grid side converters switch to voltage control of the DC-link.

11. The method according to claim 1 , further comprising: dividing the fault reference value of the d-component and/or the fault reference value of the q-component of the currents of the second group of winding sets, in respective reference portions independent of converters connected to the second group of winding sets; and supplying to each of the converters connected to the second group of converters a reference signal that depends on one of the reference portions.

12. An arrangement for controlling a multi winding set permanent magnet electrical machine in case of a fault in a first group of winding sets leaving a second group of winding sets functional, the arrangement comprising: a determining portion configured to determine a value of a torque generated by all winding sets; and a controller configured to control values of currents of the second group of winding sets based on a value of a torque to reduce a damaging torque and/or a torque oscillation occurring and/or to brake the multi winding set permanent magnet electrical machine, wherein the controller is further configured to control values of a d-component and/or a q-component of the currents of the second group of winding sets based on the value of the torque and set a fault reference value of the d-component of the currents of the second group of winding sets based on a sign of the value of the torque, wherein if sign of the value of the torque is negative, the fault reference value of the d-component of the currents is set to a negative value supported by converters connected to the second group of winding sets.